The invention relates to ultrasonic non-destructive testing devices for weld beads, particularly on pipes or tubes with a straight or helical welded seam.
In the testing of welded seams of tubes or pipes, particularly of large tubes with longitudinal or helical seams produced by submerged arc welding, ultrasonic testing devices with several testing heads are used which examine the welded seam for longitudinal and transverse defects. Longitudinal defects are faulty spots which extend in the longitudinal direction of the welded seam, for instance slag inclusions or air pockets. Transverse defects are defects which extend at right angles to the axis of the welded seam. In current testing devices various kinds of testing heads are used. Standard and angle testing heads have only one test transducer which transmits its sonic rays perpendicularly or at an acute angle to the welding seam. Double angle testing heads have two angularly arranged test transducers, one of which serves for the detection of longitudinal defects and the other for the detection of transverse defects. Alternate double angle testing heads use one transducer and a device which splits off a portion of the sonic rays and directs the split rays at an angle to the welded seam in order to detect longitudinal defects and transverse defects. Of course, these transducers also receive the sonic rays transmitted through the tube under test and converts them to electrical signals for processing.
In a known testing device four standard or angle testing heads are symmetrically arranged with respect to the seam in a K- or X-shaped arrangement. The four testing heads form a test spider which is set upon the tube and guided along the welded seam. The sound-transmitting coupling is achieved between the transducers and the tube by the supply of water into the space between the tube and these transducers. The coupling between the transducers and the tube is monitored by ultrasonic means with special control transducers in order to detect possible coupling faults such as air bubbles or loose slag.
In operation the testing heads are connected to a programmed ultrasonic impulse source and evaluating electronics. Longitudinal and transverse reflex testing beats are directed to the various transducers and appropriate receiving transducers are interrogated individually or in pairs.
In another known construction two double-angle testing heads lying on opposite sides of the welded seam opposite each other are used. Each testing head emits two sonic rays split off at 45.degree. by a quartz crystal. The ultrasonic rays which hit the welded seam perpendicularly to its longitudinal direction detect longitudinal defects and the ultrasonic rays which hit the welded seam at an angle of 45.degree. detect transverse defects.
All known devices have the characteristic that a complete examination for longitudinal and transverse defects is possible only when all testing heads are set upon the tube. The examination is therefore satisfactory in the mid-range of a tube, but fails at the beginning or at the end of the tube. When the tube is just slipped under the test spider or withdrawn from it and two of the testing heads in a K- or X- arrangement do not yet lie or no longer on the tube coupling in not present and testing cannot be performed.
This peculiarity leads to the disadvantage that the area of the welded seam at the beginning or at the end of the tube cannot be completely examined for longitudinal and transverse defects. These unexamined areas can be up to 20 centimeters (7.8 inches) in length. It was therefore always necessary to examine these unexamined areas of the welded seam at the beginning and the end of the tube manually with hand-operated ultrasonic instrucments. Manual examination requires a considerable expenditure with regard to technology and personnel.
In an older testing device of the applicant not belonging to the state of technology test transducers for the recognition of longitudinal and transverse defects are accommodated in testing heads which are arranged in a star-like test spider disposed symmetrically with respect to the welded seam. It is possible to examine welded seams for longitudinal and transverse defects with this device completely from beginning to end in a fully automatic manner. However, at least six testing heads are required for the complete examination of the welded seam. In another form of this older device double-angle testing heads having two angle transducers each, one of which transmits its sonic rays at a right angle to the welded seam (detection of longitudinal defects) and the other sends out its sonic rays at an acute angle to the welded seam (detection of transverse defects) are used. At least four testing heads are required to test for longitudinal and transverse defects from the beginning to the end of the welded seam.